1. Field of the Invention
This invention relates to a curtain applicator which is used to apply coating medium, for example color, in two layers onto a web surface.
2. Description of the Related Art
Up to now it was necessary, during the production of coated paper for use as general printing paper, for example for catalogs etc., pressure-sensitive paper, heat-sensitive paper and card paper for automatic card dispensing machines, to carry out dosing operations by way of a standard post-dosing applicator (a doctor coating unit) after applying color onto a web surface acting as base paper. Machines which are used for such applications of color are commonly known as applicators.
Up to now the most common of these applicators have used a post-dosing method with which a dosing operation is carried out after color is applied onto a web surface.
In the past few years, the color application step using a post-dosing applicator has been widely replaced by curtain applicators 100 on which, as is shown in the schematic cross-section of the main part of FIG. 4, a color ta is applied onto the web w running in the direction of the arrow by dispensing from an upper curtain head 101 in order to form a color curtain.
It should be noted that the publications cited below refer to examples of inventions which have already been disclosed and concern this application:
Cited publication 1, Japanese Patent No. S63-23; and
Cited publication 2, unexamined Japanese Utility Model Application No. H04-131700.
Two application methods are available as methods for applying two layers of color tb, tc onto a surface of the web w using the previously described curtain applicator.
In the case of the first application method, as shown in the schematic cross-section of the main part of FIG. 5, the colors tb, tc are dispensed from the upper curtain heads 201, 202 onto a web w running in the direction of the arrow in order to form individual single-layer color curtains cb, cc and to apply the colors tb, tc in two layers onto the surface of the web w. With this application method, the second layer of the color tc is applied prior to drying of the first layer of the color tb applied onto the surface of the web w.
However, with this application method the quantity of colors tb, tc dispensed from the curtain heads 201, 202 is small, for example less than 4 to 6 l/min·web width m, and the color curtains cb, cc are not very thick and cannot be used to form a normal curtain. Furthermore, blowing air over the applicator 200 causes slipping of the color curtains cb, cc, thus preventing the formation of a normal coating.
In the case of the second application method, as shown in the schematic cross-section of the main part of FIG. 6a and FIG. 6b, which is a view along the direction of arrow A in FIG. 6a, two-layer color curtains cd, ce are formed in advance by curtain heads 301, 302 and the application onto the web w running in the direction of the arrow is performed by the two-layer color curtains cd, ce.
With this application method the flow rates of the two curtains cd, ce are balanced in order to form the two-layer color curtains cd, ce, and if, for example, the flow rates of each of the color curtains cd, ce equals 5 l/min·web width m, then the overall flow rate produced is 10 l/min·web width m and the inertial force is increased, thus resulting in a reinforcement and stabilization of the color curtain and in the implementation of a stable coating.
FIG. 6b shows however that if, when using a curtain applicator 300 for the application, the curtain width cs is greater than the web width ws and the section of the curtain projecting from the web width is caught by the color recovery pan cp1, then the two colors td, te are mixed as shown in FIG. 6a. For this reason, if colors with different properties are to be applied, then it is difficult to reuse the mixed colors td, te and production losses occur.
With the first application method as described above it is possible on the other hand to recover each of the colors tb, tc individually because the color curtains cb, cc are completely separate.
Both methods have advantages and disadvantages; as such, the curtain applicators now available and used to perform a two-layer application are problematic.
What is needed in the art is a curtain applicator on which it is possible, when carrying out a two-layer application onto a web surface, to separate and recover the surplus color of the color curtain edge parts, and on which it is possible furthermore to stabilize the color curtain and perform a uniform coating.